Knee prostheses have been produced in numerous different types, the vast majority of which have distinct generally vertical and generally horizontal hinge axes. Such an arrangement of hinges does not provide for a realistic hinging movement between the femur and tibia and therefore does not sufficiently accurately replicate the function of a natural biological knee.
In an apparent attempt to overcome this shortcoming of most knee prostheses, the company MUTARS has made available a knee prosthesis in which a femoral component and a tibial component are attached by way of a ball and socket joint. The socket is formed in a generally cylindrical plastic element that is received in a transverse complementary passage passing generally horizontally through the shaped end of a femoral component. The axis of the generally cylindrical plastic element and that of a stem supporting the ball are therefore at right angles to each other.
The ball and socket formations do indeed provide for a more realistic hinging movement between the femur and tibia. However, the structural configuration of the plastic element and femoral component whereby the socket is provided has some serious drawbacks in the event that the prosthesis needs to be maintained or a component thereof needs to be replaced or serviced in some way. In such an event extensive surgery is required and, in fact, the surgery necessary is substantially as extensive as the original implant surgery.
An improved ball and socket formation is also presented by MUTARS, where the socket is a metallic cylindrical component inserted and locked in-between the intracondylar space of the femoral condyles and a half-spherical metal bearing which is attached to the tibila implant. However, metal-on-metal articulation is not desirable in implants of this nature. Design specifications of this configuration allows flexion, extension and rotation of the knee, but again replacement of worn parts requires complete dismantling of the prosthesis and major time-consuming surgery.
There is a need therefore for a knee prosthesis that is better configured for maintenance or repair purposes.